Achievements

A metal enclosure would mainly be better for making it resistant to vandalism, so is a good idea, but wouldn't do much to make it more resistant to being cracked by a determined opponent, since they will be able to get in one way or another. The tamper switch is a good idea though - the way to use it is to program the device to instantly delete its cryptographic key if the tamper switch is activated or the box loses external power (you'd need to add e.g. a large capacitor to give it enough time after losing external power). Then even an advanced adversary with the capability to snoop directly into the electronics as they are running may inadvertently thwart their efforts just by opening it up. You'd then have to have a way to manually reset the system and send the outside box a new key ...

A metal enclosure would mainly be better for making it resistant to vandalism, so is a good idea, but wouldn't do much to make it more resistant to being cracked by a determined opponent, since they will be able to get in one way or another. The tamper switch is a good idea though - the way to use it is to program the device to instantly delete its cryptographic key if the tamper switch is activated or the box loses external power (you'd need to add e.g. a large capacitor to give it enough time after losing external power). Then even an advanced adversary with the capability to snoop directly into the electronics as they are running may inadvertently thwart their efforts just by opening it up. You'd then have to have a way to manually reset the system and send the outside box a new key (after confirming that no covert additions have been made to capture it during this process) for initial setup, for when the power goes out, or for when you need to service the system.